Since, the first introduction by Adleman in 1994 (Adleman, Science 266, 1021-1024 (1994)), DNA logic gates have been considered as the future of computation technology where their small size is a distinct advantage over the conventional top-down semiconductor technology. They also exhibit important applications in life sciences as smart sensing and diagnostic platforms owing to the unique properties of DNA such as self-assembly, specific recognition and conformation modulation upon exposing to external stimuli (i.e. metallic ions, proteins) (Elbaz, J., et al. Nat. Nanotechnol. 5, 417-422 (2010)). Along this line, DNA-based systems (e.g. DNAzymes (Bi, S., Yan, Y., Hao, S. & Zhang, S. Angew. Chem. Int. Ed. 49, 4438-4442 (2010)) molecular beacons (Park, K. S., Seo, M. W., Jung, C., Lee, J. Y. & Park, H. G. Small 8, 2203-2212 (2012)), guanine-rich oligonucleotides (G-quadruplexes) (Li, T., Wang, E. & Dong, S. J. Am. Chem. Soc. 131, 15082 (2009)), aptamers (Liu, X., Aizen, R., Freeman, R., Yehezkeli, O. & Willner, I. ACS Nano 6, 3553-3563 (2012)) have been devised on different nanometer scale carriers such as graphene and graphene oxide (Wang, L., et al. ACS Nano 6, 6659-6666 (2012)), solid-state nanochannels (Jiang, Y., Liu, N., Guo, W., Xia, F. & Jiang, L. J. Am. Chem. Soc. 134, 15395-15401 (2012)), quantum dots (Freeman, R., Finder, T. & Willner, I. Angew. Chem. Int. Ed. 48, 7818-7821 (2009)), and gold nanodisc arrays (Witlicki, E. H., et al. J. Am. Chem. Soc. 133, 7288-7291 (2011)) for various logic gate operations and biosensing applications. Those DNA logic operations mostly rely on fluorescence and enzyme cascades to generate “ON” or “OFF” output signals which involve complex handling and analysis procedures, thus restricting the performance and applications of the sophisticated logic devices. In addition, it still remains very challenging to realize a label-free and switchable DNA logic gate-based biosensing platform that can selectively respond to extremely low concentration of the chemical and biological stimuli.
There is a recent emergence of plasmonic metamaterials capable of providing high electromagnetic enhancement (hot-spots) for surface enhanced Raman scattering (SERS) (hereafter called MetaSERS) (Xu, X., et al. Nano Lett. 11, 3232-3238 (2011)). The most prevailing SERS sensors are based upon chemically synthesized colloidal nanoparticles (Nie, S. & Emery, S. Science 275, 1102-1106 (1997)), but they have the disadvantage of poor reproducibility. Recent advances have employed a variety of top-down fabrication techniques which enable large-scale and reproducible patterns for SERS substrates ranging from bow-tie nanoantennae (Hatab, N. A., et al. Nano Lett. 10, 4952-4955 (2010)) to asymmetric Fano resonance structures (Zhou, W. & Odom, T. W. Nat. Nanotechnol. 6, 423-427 (2011)). In contrast to these structures, plasmonic metamaterials have recently been demonstrated to offer effective ways to tailor the concentration of light to form desired hot-spots by controlling the size and shape of plasmonic structures (Schuller, J. A., et al. Nat. Mater. 9, 193-204 (2010)). By properly designing the micro- or nano-scaled metallic sub-wavelength structures, e.g. “split ring resonators (SRRs)”, one can tune the operating frequency of metamaterials from microwave (Shelby, R., Smith, D. & Schultz, S. Science 292, 77-79 (2001)) to visible regime (Xu, X., et al. (2011)). However, most of metamaterial-based biosensing focuses on the localized surface plasmon resonance (LSPR) shifts induced by absorbed molecules (Liu, N., Tang, M. L., Hentschel, M., Giessen, H. & Alivisatos, A. P. Nat. Mater. 10, 631-636 (2011)), in which the shift depends on the effective refractive index of the target molecules thus exhibiting no chemical fingerprints. Recently, Fano-resonant asymmetric metamaterials have been introduced to demonstrate the ultrasensitive sensing and identification of molecular monolayers by tuning the resonant peak towards (away from) protein's vibrational fingerprint and monitoring the infrared reflectance spectra (Wu, C., et al. Nat. Mater. 11, 69-75 (2012)).
Structural and functional information encoded in DNA combined with unique properties of nanomaterials could be of use for the construction of novel biocomputational circuits and intelligent biomedical nanodevices. However, at present their practical applications are still limited by either low reproducibility of fabrication, modest sensitivity, or complicated handling procedures.
Bivalent mercury ions Hg2+ are the most stable inorganic forms of mercury contaminant in environment and food products and are responsible for a number of life-long fatal effects in human health such as kidney damage, brain damage, and other chronic diseases (Clarkson, T. W. & Magos, L. Crit. Rev. Toxicol. 36, 609-662 (2006)). According to the United States Environmental Protection Agency (EPA), the maximum amount of mercury should be lower than ppb and ppm levels for drinking water and food products, respectively (Clarkson, T. W. & Magos, L. (2006)). However, because mercury has a strong bioaccumulation effect through the food chain (Morel et al. Annu. Rev. Ecol. Syst. 29, 543-566 (1998)), there exists a great demand and also a significant challenge for development of a method that allows facilely monitoring the concentration of mercury below the defined exposure limit level.